Soil encountered close to the earth""s surface is one of the most important engineering materials in the fields of civil, highway, airfield and architectural engineering. The load bearing capacity of the soil supporting highways, airfield runways and other pavement systems is of immense importance to the integrity of the pavement. This load-bearing capacity, or soil stiffness, changes from time to time and can vary from place to place within a given area.
Soil stiffness is the degree of resistance to deformation upon loading. Soil will generally undergo at least a certain degree of deformation when subjected to load. Overall soil deformation is due mainly to distortion and compression. Distortion is the elastic deformation of soil solids. Compression is the volume change resulting from expulsion of moisture from pores (voids), and is called consolidation. The extent and time-dependence of, and the degree of recovery from, deformation is primarily dependent upon the soil""s properties, existing stress conditions and stress history.
Soil properties in turn are determined by a variety of complex interrelated factors, including without limitation soil composition particle size, particle-size distribution, and the like), weight-volume relationships (solids/moisture/vapor/voids proportions, density and the like), engineering properties (cohesion, consistency, structure, permeability, water/soil interactions and the like) and in-situ stresses (vertical overburden stress, hydrostatic stress and the like).
The stability or load-bearing capacity (capability) of the pavement of airport runways, highways and other pavement systems is determined in significant part by the load-bearing capacity of the underlying (subpavement) earth or soil, which may deteriorate over time due to environmental and stress influences on soil properties. For instance, changes in soil load-bearing conditions due to changes in moisture content and/or repeated loading over time are well recognized in engineering fields. In addition, certain pavement systems such as runways and highways typically endure repeated severe loadings on a daily basis.
The proper determination of existing bearing-load capacities of soil-supported pavement systems requires that the existing soil conditions be defined and evaluated. Conventional soil-structure modeling is based on the results of laboratory testing of individual localized soil samples. Laboratory test methods, however, are severely disadvantaged because the test conditions and the soil sample (specimen) are not representative of in-situ conditions. Absent are (a) in-situ overburden stress, (b) in-situ soil interactions, and the like. Further many if not most soil samples have been disturbed to some degree during sampling and handling. A true composite soil stiffness determination can only be determined using actual stiffness data of in-situ soil conditions at varying depths (varying subgrade conditions). In addition, while soil samples from individual lifts of soil placement can be obtained with relative ease before and/or during construction of the pavement system, thereafter the overlying structure generally precludes sampling of the supporting soil by nondestructive methods.
Another known method for determining composite soil stiffness is the use of plate bearing tests on the surface of soil layers after placement and compaction of each, prior to placement of another layer above. These results, while meaningful, only provide results at that point in time, and do not provide localized information regarding individual areas at specific depths of the soil layers.
Soil stiffness determinations require (a) the application of a predetermined surface force and (b) the measurement of the resultant deflection or vertical deformation of the soil. Apparatus for applying a predetermined surface force are well known. Apparatus for measuring resultant deflection at the surface are also known. The challenge is the instrumentation and methodology needed to obtain actual stiffness data of in-situ soil conditions at varying depths to obtain the data necessary for the definition and evaluation of existing soil conditions, and then to properly determine existing bearing-load capacities of the overlying pavement system.
The most direct method of measuring composite and individual soil layer stiffness and deflections is through the use of a multi-depth deflectometer (xe2x80x9cMDDxe2x80x9d). A known type of MDD utilizes a linear array of vertically linked displacement transducers for measurements at multiple depths. The transducers used in that linear array are known as xe2x80x9clinear variable displacementxe2x80x9d transducers (xe2x80x9cLVDTxe2x80x9d) and are commercially available. They are fabricated as modules some four to six inches in length which are themselves positioned in vertical alignment within a bore hole at the desired subsurface measuring levels. Interconnecting rods link the LVDT""s one above another within the bore hole. The soil-gripping mechanism of LVDT""s are sidewardly extending ball bearings that require activation with a custom tool after subsurface placement. The lengths of the linked LVDT module preclude the measurement of layers that are closer than about six to twelve inches (that is, the distance between ball-bearing sets of adjacent and closely spaced LVDT""s). Another drawback inherent in this known MDD is that the failure of just one of the linked LVDT""s in a strand (in series electrical arrangement) results in the total failure of the instrument. Any displacement transducer can, for instance, fail from overload. The LVDT""s of this known MDD are also at risk of failure from contact with, or submersion in, water because they are exposed to subsurface water. The proposed solutions to the water-exposure risk are either hermetically sealing of the subsurface apparatus, which would be a costly addition, or the very expensive use of fully submersible transducers at a cost of about $1,000 apiece. The standard LVDT""s are relatively expensive and other MDD components are expensive and difficult to install. The installation time reported in the literature is twelve hours over two separate days. (The anchor must be installed a day before the other components.) This MDD cannot be prefabricated because activation of ball bearings must be performed after subsurface placement and the top of the LVDT being activated must accessible by a special tool. Therefore a higher-positioned LVDT cannot be connected to the strand until activation of the LVDT positioned below.
It is desirable to measure soil stiffness at multiple depths using instrumentation and methodologies that provide rapid and preferably continuous measurements through an automated system to reduce the time and the costs of taking measurements, and to avoid operator exposure to traffic.
Preferably the instrumentation is relatively inexpensive to fabricate and inexpensive to install. Preferably the instrumentation can be readily installed either during construction of the pavement system and or during post-construction time periods with substantially nondestructive installation methods that do not jeopardize the pavement system. Preferably the instrumentation is a completely prefabricated assembly so that field installation is rapid.
Preferably the instrumentation includes multiple transducers, each of which will continue to function despite the failure of another. Preferably the electronic components are both accessible for adjustment if there is a disruption or failure, for instance due to an overload, and preferably the electronic components are not exposed to subsurface water without the expense of hermetically sealing the down-hole components. Preferably only simple mechanical components of the instrumentation are installed at subsurface positions, while the more delicate electrical components are positioned in more readily accessible locations.
Preferably the instrumentation can be readily fabricated with a virtually unlimited number of transducers for concomitant deflection-measurements at a virtually unlimited number of depths. Preferably the measurement depths can be relatively close to one another, for instance such as within about one or two inches of each other.
The deflections being measured are typically extremely small, for instance often no more than 0.01 inch or even less. Another challenge is to provide the apparatus and methodology that provide the desirable characteristics mentioned above while accurately measuring such minute deflections.
The present invention provides a multi-depth deflectometer comprised of a head member that normally is disposed at the very mouth of a bore hole, and an elongated tail member extending down through the bore hole. Displacement transducers are removably positioned within the head member which can be opened from the surface. The transducers can be easily placed in or removed by hand. The first trackings of subsurface deflections are done through mechanical deflection anchors located at desired measuring levels within the tail member. The deflection anchors have transverse-bite actuators and are each separately in vertical-actuation communication with one of the transducers. The present invention is also a method of implementing the MDD of the invention.